Mass separator



Jan. 26, 1954 J. G. BACKUS 2,667,582

MASS SEPARATOR Filed Feb. 27, 1948 2 Sheets-Sheet l [III/IIIIIIIIIIIIIIIIIII/IIIIIIIIIIIIIIIIIIIIIIIII/j r IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII'IIIIIIIII IIIIIIIII JNVENTOR. JOHN G SAC/(U5 BY a ATTORNEY .ent in an i evacuated space.

Patented Jan. 26, 1954 UNITED STATES PATENT OFFICE MASS SEPARATOR John G. Backus, Los Angeles, Calif., assignor'to the United States of America as represented by the United States Atomic Energy-Commission- Application February 27, 1948, Serial No. 11,729

'l' Claims. 1

The present inventionrelat'es to arrangements and apparatus for' magnetically and electrically influencing a beam of ions in order toresolve the 10115 into-separate, distinguishable groups .of like lmass'sspectrometersFand.calutrons..

Moreroriless conventional mass separators have beeniprev-iously utilized to. analyze the gases pres- However; such-apparatus is designedtoidentify only the lighter elements and thus maybe made quite small in size: In order to provide satisfactory resolution of any of the elements; light or heavy, numerous-hitherto disregarded factors must lee-considered. Careful observationreveals that a conventional ion source produces ions having an energy variation of from 3D. to 50 volts. A practical ion source mustcombine simplicity and ruggedness; Thus, even though an ion-source couldbe designedhaving amore uniform ion energy, it would be at the expense-of a reduction'in the above two prerequisites; In order to obtain satisfactory resolution of heavy elements with such an initial energy'spread it' would be necessary to use high accelerating potentials andthuslargeradii of curvatur 'and large -magneti'c-fields if only conventional magnetic focusing were used. Thus to'obtain-a-high quality of resolution and yet maintain apractical size, other means of focusing are necessary;

Theproper-interaction ofel'ectric and magnetic fields-Will produce'a focusing effect suflicient to overcome any oftheabove-mentioned difliculties. The generalprinciple-of this type of focusing was d'escribedby Dempster and Bartky'in the Physical'Review; v0lume 33, page 1019, however the instant-invention combines anovel arrangement of elernents'and impressed potentials in order to provide a practical apparatus utilizing this prin ciple.

I't'is therefore anobject of this invention to provide a velocity-focused mass separator.

Another object of thisinvention is to provide amass: separator having an extremely high degree of resolution of the ion beam;

Another object of this invention is to provide an apparatus suitable for. use in the physical assay analysis of heavy ions.

Another object I is' to provide inianiappanatusof the class described an electric field capable ofiimproving: the: iombeamiocus.

A further objectis to; provide an apparatus of the class described utilizing magnetic and electric focusing fields and means to counteract defocusin'g efiects in the region of the sourceand receiver.

A further object is to providea mass separator capable of focusing ions having. small energy variations.

A still further object is to provide means within a mass separator whereby a divergent ion heam may be focused.

In order to apply -the principles of velocity focusing, various. factors must be considered, among which are: magnet field strength, electric field strength, and the relative distribution and interaction of the magnetic and electric fields. Certain other practical considerations are also of importance, such as; allowable inhomogeneities of magnetic and electric fields, slope of magnetic and electric fields, and variations in the i011 direction, velocity, and mass.

The manner in which the above factors are dealt with is fully discussed in the following description of construction and operation of a mass spectrometer for example, inconjunction with which there areincluded-four figures: Figure l is a side view 1 of asimplified. schematic representation of the mass spectrometer, showing portions cut away; Fig. 2 is a sectional view of Fig. 1 taken along plane 2-2; Fig. 3 is-a diagram of one preferred embodiment of the electrical system associated therewith; and Fig. 4 is a diagram of the apparatus including appropriate reference symbols used incon'junction with the theoretical considerations-presentedlierein.

Referring more specifically to the included drawings, it is to be not-ed that apparatus proper is contained in a tank l which is evacuate-d. Withi-n-theboundaries of the tank I a uniform magnetic field is maintained which is perpendicrular. to the plane-of the-drawing and directed upward out of the paper. lens are formed in a conventional ionization chamber 1 I comprising an anode I2 and cathodes it. An accelerating plate l4 including anaperture t serves to remove the ions from the source, and a defining slit 5 inplate I15 .col'limates the ion beam. A receiver I 6; serves 'to collect theion beamthrough acollector slit Q'in plate lfi; As'shown, the receiver l-.6 isconnectedto :a cathode ray oscilloscope thus providing a pictorial: representation of the ion beamihowe-ver, various-othertypes of receivers woul'clube: suitable, such as a photographic plate for the analyzation of radioactive isotopes. Two .concentric'1-cylind1ica1Eplates: i l. and t8 ar --arranged parallel to the magnetic field and disposed on opposit sides of the crescent-shaped region 23 in which an ion beam projected from the ion source I l to the ion receiver l6 travels. The plates I! and 18 are mounted upon nonconducting plates 2! and 22 as shown in Fig. 2, thus defining the annular region of ion travel 23.

An electric field is maintained within the region 23 by the application of potentials to the plates i1 and I8, plate It being the more positive of the two. Referring to Fig. 4, the ions traversing this region are subject to a magnetic force inward toward the center of their orbit and an electric 7 force outward from the center 0. For a particle of mass m and charge 6 to describe a circular orbit of radius p and center 0, w must have the centripetal force acting on the particle equal to the difference between the force applied by the magnetic and electric fields, or

this will be zero for Hence a particle entering the field with the velocity given above will describe a circle of constant radius p. It may also be noted from the above that for a constant radius orbit; that is, the magnetic force inward is twice the electric force outward, which condition may be attained by adjusting the voltage applied to the plates 51 and I8 and the strength of the magnetic field. V

The orbit of a particle entering the system with slightly different values of m and V (accelerating potential) from those giving a circular orbit has been calculated, and it has been determined that the foci exist at 7 or 12717' which incidentally is the same result as for the purely electrostatic case of a radial electric field. Thus the receiver [6 and source I l are constructed so as to include an angle of 12717 there'between. Furthermore the radial position of the focus varies directly as the mass which is the same as for the 180 magnetic focusl Further research on the problem. of; ion source energy variations and initial particle angle leads to the following relationship:

where a is a small angle variation from the constant radius path, see Fig. 4

'6 is a small energy spread between ions from the source A is the atomic weight of a particular element to be analyzed w is width of entrance slit 3 is width of exit slit The above relationship assumes that a resolution of individual isotopes or mass units is desired; however, if such is not the case, the left-hand term of the equation may be made less than any other desired resolution. The approximate value of 6 (energy spread) may be either calculated or observed and thus the maximum allowable a determined.

An investigation of the effects of magnetic field inhomogeneities results in the conclusion that the ion path must everywhere be at least a gap length away from the edges of the pole pieces. This however does not cause undue hardship as the lateral spread of the beam is maintained within close limits and thus the gap (distance between magnet pole pieces) may be quite small.

Inhomogeneities in the electric field pose a more difiicult problem. It may be shown that accelerating voltage deflecting voltage 26 thus the voltage ratio is a constant for the ap paratus and with this ratio the condition for velocity focusing is satisfied for all mass, it remaining only to adjust the magnetic field to bring different masses to the focal point. However, the electric field set up between plates l1 and I8 is not uniform at the extremities of these plates in the vicinity of the source H and receiver [6. With these electric field irregularities it would be necessary to materially increase the width of the plates I7 and It in order for the ion beam not to be affected by the irregularities. Such an increase in plate width would necessitate a much larger magnet gap which is an undesirable requirement. However, the present invention circumvents this difiiculty in another manner. A number of curved strip electrodes, such as 31 and 32, are placed between the plates 11 and I 8. These strip electrodes are arranged along concentric circular arcs and are divided into two sets, one of which is on one side of the ion beam and the other on the opposite side of the beam, whereby pairs of electrodes of equal radius are disposed on opposite sides of the beam along the magnetic field. The two electrodes of each pair are electrically connected, and all of the pairs, as well as the plates H and I8, are connected to appropriate points on a voltage divider so as to achieve the desired distribution of the electric fieldin the region traversed by the ion beam. If V1 is the voltage across the plates 11 and l 8, and n the number of strips, the proper voltages to apply to potential dividers will be plate 11.

With reference to the electrical circuitassoracemes ciatecl --withthe apparatus Fig. 3 1 shows a :power supply applying a positive potentialito the source ianode f 12,-and equal negative potentials to the cathodes :13. *ment .1 the accelerating electrode 4 l4 1 is grounded .and a high positive accelerating potential applied to'the anode iz by a power supp1y142.

In this particular emboditained therebetween. Equal voltages of opposite sign are applied to the plates H and 5'8 which means that the midpoint between these plates :should be at zero potentialand thusthe center pair of strip electrodes are grounded insuringga :zero potential thereon.

As previously mentioned, various means may ,be employed to view therresults, of-;w-hich only one preferred embodiment is depicted herein.

.-'-As shown, asaw-tooth generator efidmpresses a voltage of saw-tooth wave formupon; the-deflecting voltage. This potential variation on the defleeting plates causes the ion beam to sweep across the receiver. The receiver is connected to the vertical deflecting plates of a cathode ray oscilloscope and the saw-tooth voltage is impressed on the horizontal deflecting plates thus producing a trace of ion concentration vs. ion mass on the fluorescent screen of the oscilloscope.

A brief review of the operation of the apparatus shows that positive ions are formed at the source II and attracted from it by the relatively negative potential on the accelerating electrode M. The magnetic field exerts a force on the ions at right angles to their direction of motion, thus forcing them to assume a semicircular orbit. The ions thus travel from the source I4 through the crescent-shaped region 23 to the receiver It.

An electric field is maintained between the concentric circularly cylindrical plates l1 and H3 in such a direction as to urge the ions away from the center of their orbit. With the proper relationship maintained between the electric and magnetic fields, as given above, the efiects of energy variations among the source ions is overcome and ions of like mass-to-charge ratios are focused at the receiver. There is also provided a number of pairs of strip electrodes, arranged as shown, which are attached to a voltage divider 43, thereby insuring the proper distribution of the electric field within the region of ion travel 23. Thus with a particular value of magnetic and electric fields all ions of a particular mass-tocharge ratio are focused at the receiver. A change in the value of the electric field causes ions of another mass-to-charge ratio to focus at the receiver.

A varying voltage, preferably of saw-tooth wave form, is impressed on the voltage divider 43 thus varying the electric field and sweeping the ion beam across the receiver, focusing ions of like mass-to-charge ratio in succession. As the receiver is connected to the vertical deflecting plates of a cathode ray oscilloscope and the saw-tooth voltage to the horizontal plates, a trace of ion magnitude vs. mass-to-charge ratio is obtained on the fluorescent screen of the oscilloscope.

Calibration of the screen in accordance with the magnitude of the saw-tooth voltage makes it possible to immediately identify the elements and isotopes presentxin;;the..sonrce.1 material. and

-compare1theiri relative. abundance.

Q'Ihe:foregoing'presentsa velocity focused mass v.spectrbmeter.iniwhich a1radial electric field is :5 --maintained.-perpen dicular:toithe magnetic field .andito the ion-path. iWlthlthiS novel apparatus ait ismowpossible forions of difiering initialve- -looity-..and. directionztoqbe precisely focused at-a receiver. Further advantage. lies in the utilization J0 *of stripielectrodes..whereby.the problem of de- ---foc.using posed .by the presence of the electric .-.-field 1-.iS30V81COIIl6. Jul-addition to the advancevmentsspresentedzin the mass spectrometer proper, there is included a dependable, simply-operated -means-for;determining-the physical properties of the: separatediions. The combination provides a .noveliandpracticaliapparatus-capable of identify- .ingwhatsoeverion, isotope, or element be desired.

.Although this invention has been describedwith reference to a mass spectrometer, it will be ob- .vious' to: one skillediin. thev art that ready adaptaztionttoacalutron useinvolves at most merely. ad-

justing thepotential:impressed upon the voltage .tdivider. "jfiherefore, it is understood that no .25 limitationisfi expressed or impliedexcept as may .be comprehended;- by the terms. of the. appended claims.

What is claimed is:

1. A mass separator comprising a source of 39 ions, an ion current receiver, a beam of ions traversing an arcuate path therebetween and means including at least four curved electrodes parallel to and symmetrically disposed about said ion M beam path for establishing a uniform, cylindrical electrical field for focusing ions emitted from said ion source at said ion receiver.

2. A mass separator comprising means establishing a magnetic field, a source of ions, an ion current receiver, a beam of ions traversing an 40 arcuate path therebetween and means including a plurality of pairs of curved electrodes disposed parallel to said ion beam arc for establishing a radial electrical field influencing and perpendicular to the path of ions projected from said source through said magnetic field to said receiver, the two electrodes of each pair being disposed along a line parallel to the magnetic field and on opposite sides of the space through which the ions are projected.

3. A mass separator as claimed in claim 2 further characterized by means to apply a variable potential to said electrodes thereby varying the orbits of said ions, and further means providing r a visual indication of the ion mass and relative 0 abundance of each different mass.

4. A mass separator comprising a source of ions, an ion current receiver, and means for transmitting ions along curved paths from said source to said receiver, said means including two sets of curved electrodes parallel to said ion paths and disposed on opposite sides of said paths in planes substantially parallel to the planes of said paths for establishing an electric field substantially perpendicular to said paths.

0 5. A mass separator comprising a source of ions of different mass-to-charge ratios, an ion current receiver, velocity-focusing means including a plurality of electrostatic deflecting plates dis 7O posed on opposite sides of a region traversed by ions from said source to said receiver for focusing ions in a wide velocity range upon said receiver, a plurality of electrodes disposed between said plates, said plates and electrodes having substan- 75 tially the same curvature as said ion paths and 1 source connected across said voltage divider for successively focusing ions of difierent mass-tocharge ratios at said receiver.

6. A mass separator as claimed in claim 5 further characterized by means including a fluorescent screen whereby the relative abundance of each group of ions of like mass-to-charge ratios is visually represented.

'7. In an electromagnetic mass separator including an ion source and means projecting a beam of ions therefrom, an ion receiver, and means establishing a magnetic field through the region between said source and receiver substantially perpendicular to said beam of ions, whereby said ions are constrained to travel an arcuate path; the improvement comprising a pair of curved electrostatic deflecting plates disposed parallel to said magnetic field on opposite sides of said ion beam and in substantially parallel relationship thereto, a plurality of pairs of curved electrodes substantially parallel to said ion beam path and disposed intermediate said plates with one of the electrodes of each pair lying in the magnetic projection of the other, potential supply means connected across said plates and electrodes for establishing a radial electrostatic field through the region traversed by said ion beam, whereby ions are focused at said receiver substantially independent of their initial velocity. JOHN G. BACKUS.

References Cited in the file of this patent UNITED STATES PATENTS Name Date Hipple Oct. 5, 1943 OTHER REFERENCES Number 

